Will a single ductless mini-split heat pump be able to distribute its air evenly throughout my house?
I am in the middle of a deep energy retrofit of my 1,200 sf New Mexico house, and a manual J calculation of the projected loads once my projects are completed shows an estimated heating and cooling load of only around 13,000 BTUs each, compared to the current baseline of 41,000 heating / 28,000 BTUs cooling.
Since this is such a small number, my existing 125,000 BTU furnace will become even more ludicrously oversized than it is now. So on everything is done, I’d like to replace it with a single ductless mini-split heat pump.
Here’s my quandry: the numbers alone say that a single-headed heat pump unit should easily be able to handle both the projected heating and cooling loads. But I’m worried about getting the conditioned air consistently into the three bedrooms, which in typical ranch house fashion, are on the other side of the house from the living area, accessed through a hallway. The living area will probably be fine, but I’m worried that the bedrooms might not get enough of the conditioned air, even with a sealed and well-insulated building envelope (R-29 walls, R-50 ceiling, R-20 footing insulation, good windows).
If a single unit will be enough, where should I put the unit? In the living area above the entrance to the hallway? If a single unit won’t be enough, would a double-headed unit with a single outside compressor work? Where should I put the other unit? Am I overthinking things?
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Here's an article that addresses your concerns:
https://www.greenbuildingadvisor.com/blogs/dept/musings/just-two-minisplits-heat-and-cool-whole-house
Nathaniel,
In addition to the article that Kevin linked to, there are many other GBA articles that address your question. Here are some links for you:
Two Years With a Minisplit Heat Pump
Report on Our Ductless Minisplit Heat Pump
Loving My Minisplits
Minisplit Heat Pumps and Zero-Net-Energy Homes
Practical Design Advice for Zero-Net-Energy Homes
Putting the Duct Back in Ductless
Thank you, guys. I'd already read about half of those, and the other ones filled in a lot of gaps in my knowledge. I'm considering going with a double-headed model with two 8,000 BTU heads, one supplying the open living area and the other supplying the hallway that leads to all the bedrooms. Or would folks recommend two single-headed units instead? Is the SEER lower on multi-head units? Either way, the air should be thrown in two directions from the center of the house, and the bedroom side of the house is a separate zone from the main living area. Bedroom doors will be open. :) Here's my layout idea:
How about one head at the other end of the hall? Or better yet to me would be to eliminate the kitchen/living room partition and put one head there blowing down the house and down the hall. That to me would be ideal.
I could actually put it on the tip of the partition wall; there's a 31-inch wall on the end of it that I didn't put on the diagram, and it faces down the hallway. Of course, if I put it there, I have no idea how I would adequately attach it to the exterior unit.
Based on where you have the heads located on your drawing putting a head on the tip of the partition wall would involve the same way of connecting it to the outdoor unit. The lineset would have to go up through the attic somehow in either case.
From an ease and cost of installation standpoint having the indoor head on an exterior wall is the easiest and generally cheapest. If the indoor unit is located on the exterior wall to the living room side of the partition wall so that the air is directed towards the hall, there should be enough air movement to get to the bedrooms. That is, if the bedroom doors are left open and the wall at the tip of the partition wall isn't in the way.
Not all indoor units have vanes that can direct the air left/right though, so if the unit has to be off to the side (not aimed directly at the hall) than I would make sure that the unit has lateral vanes.
If you do go with two indoor units I would stick with a single outdoor unit. Your heating load will be small enough that two outdoor units would be overkill.
Since you are in NM I doubt you need the hyper heat for low temperature performance, in which case they have ceiling cassette and other styles that can handle multiple rooms
As long as you make your decisions before you have the sheetrock up, interior wall installs are no big deal
You could put the unit on the end wall of the public space and build the lineset/wiring in for a second one at the far end of the hallway[which you won't need but will make you feel better]